There are many environments where the sterilization of articles is of critical importance. Hospitals and medical offices must sterilize certain equipment before that equipment can be utilized with surgical procedures or in sterile environments. Also food service environments must take care that their utensils and equipment are sterilized in order to prevent subsequent possible infection of later users.
Commonly the equipment, instruments or materials that are to be sterilized may be subjected to the necessary parameters to achieve sterilization including the interrelated parameters of time, temperature, steam, dry heat, chemicals such as ethylene oxide gas or radiation dosage. The sterilization process is designed to kill living organisms which might otherwise contaminate the goods being sterilized. Steam sterilization ranges from 121-132 degrees Centigrade with exposure times of three minutes at 132 degrees Centigrade to 30 minutes at 121 degrees Centigrade. Ethylene oxide sterilization ranges from 30 to 65 degrees Centigrade with exposure times of one hour to four hours. Dry heat sterilization typically is two hours at 180 degrees Centigrade.
Since it is critical in many environments that the sterilization be complete, frequently sterilization monitors are utilized. Typically, a small vial, or other container, is inserted into the sterilization unit in order to check upon the completeness of the sterilization process. The vial typically will contain living organisms that are provided nourishment and an environment for rapid growth after being subjected to the sterilization process.
The vial can then be checked for growth of the organism after a incubation period. The incubation allows any organism surviving the sterilization process to grow to a detectable level. Survival of the organism indicates an inadequate sterilization process.
In order to properly detect living organisms, typically a substrate material is added to the vial for the living organisms to react with causing the substrate to change color.
Typically, the reading or analysis of the color change has been done manually with observation by the human eye. However, in one sterilization monitor process, a potentially luminescent substrate material is added to the vial for the living organisms to react with. After allowing for a suitable reaction period, the vial is subjected to a fluorescent light, typically an ultraviolet wavelength light, which excites the invisible to the naked eye fluorescence to appear as visible light. The appearance of visible light in the vial when subjected to an exciting fluorescent light would indicate inadequate sterilization, due at least in part to a shortened time period required for the growth of the living organisms, more automated fluorescence detectors are to be utilized.
Further to more rapidly determine the completeness of the sterilization process, it is desired that fluorescence from the luminescent substrate in the vial be measured at a very low threshold, approximately 0.1 picowatt.
U.S. Pat. No. 5,073,488, Matner, Foltz and Woodson, Rapid Method For Determining Efficacy Of A Sterilization Cycle And Rapid Read-out Biological Indicator, assigned to the assignee of the present invention, discloses a method of developing the fluorescing indicator used in a fluorescence detection apparatus.
Unfortunately, the ultraviolet light source typically used tends to be very unstable due in part to variances in the gas ionization at one or more of the cathodes located at each end of the ultraviolet light source. Further, localized heat variations and emission fluctuations cause the output of the ultraviolet light source (lamp) to contain many independently varying harmonics of the power line frequency. The overall crest-to-trough ratio fluctuates similarly. The use of DC coupled amplifiers with low pass filtering to remove these fluctuations is unacceptable because the DC drift would be well in excess of the signal level being measured.
Prior art fluorescence detectors sometimes utilize dual channels to attempt to adjust for these fluctuations. However, a further problem exists when dual channels are utilized or when the target measurement is taken at a different time from the reference. Since, the signal being measured is so small, minor variations in components between dual channels, even with costly matched components, and between different times of measurement can result in a complete "swamping" of the signal to be measured. The result can easily be an inaccurate measurement. In order to compensate for the inherent inaccuracies in measurement, the "growth" period must be expanded (or not reduced) in order to ensure that living organisms, if present, will be detected.
U.S. Pat. No. 4,626,684, Landa, discloses an apparatus for rapidly conducting fluorescence measurements. The electronics of the apparatus utilizes dual channels. One channel detects the fluorescence of the sample. The other channel detects the fluorescence of a reference. The "background" reference fluorescence is subtracted from the signal sample in order to account for the background. However, in order for the reference to be effectively canceled, the dual channels must be exactly matched.
U.S. Pat. No. 4,668,868, Noller, discloses a fluorescence detector for performing fluoroimmunoassays of biological specimens. The light energy from the sample is used to excite a photovoltaic cell. Temperature compensation is provided by a second photovoltaic cell from which light excitation is eliminated. A push button calibrator also used to provide a bias voltage from the voltaic cell with zero input signal which is memorized. A significant disadvantage of the system of Noller is the lack of a reference value. Since fluctuations in the intensity of stimulating radiation may greatly affect the amount of visible light output apparently perceived can vary greatly.
U.S. Pat. No. 5,030,832, Williams et al, Apparatus For Detecting Fluorescence Of A Luminescent Material, assigned to the assignee of the present invention discloses the construction and operation of an electronic instrument for go/no-go measurement of a fluorescing sterilization monitor. An electronic circuit follows the power fluctuation of the excitation light source and thus follows the variations of the light output. Calibration of this system is manual.
U.S. Pat. No. 5,063,297, Hardenbrook et al, Apparatus For Detecting Fluorescence Of A Luminescent Material, assigned to the assignee of the present invention, is a continuation-in-part of the Williams '297 patent and discloses a single channel detector circuit.